Short arc lamps, especially Xenon lamps, have been used in many applications, including camera strobes, analytical instrumentation, surgical illumination, theatrical lighting, and laser and machine vision. In spite of the availability of other more convenient and low cost light sources such as LEDs (light emitting diodes), Xenon lamps are still currently used in some niche areas because they have certain unique properties that other light sources cannot provide. These include high brightness, high power, high UV (ultra violet) light content, a wide continuous spectral distribution with excellent color balance and spectral flatness in the visible region, long life and stable spectrum over the life of the lamp.
Xenon lamps have two operation modes, namely DC and pulsed mode. The DC operation mode generally has a better arc stability and substantially longer lamp life than the pulsed mode. However, this mode of operation is not ideal for photography which only needs a short flash of illumination light while a photo is being taken. As for the pulsed mode of operation, the combination of wide spectrum and color balance with the ability to produce short pulses of high brightness light has made Xenon lamps particularly suitable for biological photography, enabling excellent color projection and high-quality flesh tones.
In this respect, short-arc flash lamps with an arc spacing of typically 1-3 mm are especially unique because they can provide pulses of high intensity light and brightness that other light sources cannot match. The high brightness and intensity is particularly desirable for superior camera performance. In addition, a short-arc flash lamp can also solve the problems related to motion of a living biological sample, such as a human eye, and hence eliminate blurring of the obtained image. Furthermore, the wide spectral distribution of Xenon flash lamps also makes them ideal for applications requiring light in specific spectral regions, such as red-free images and Fluorescein Angiography. The required spectral region is obtained by placing different types of optical band pass filters in the illumination and/or detection light path.
In its simplest form, a Xenon flash lamp is composed of a sealed glass tube with an electrode at each end and is filled with pressurized Xenon gas. A typical electronic flash circuit consists of four parts: (1) power supply, (2) energy storage capacitor, (3) trigger circuit, and (4) flashtube. FIG. 1 shows a typical Xenon flash lamp discharge circuit with a trigger circuit. The energy storage capacitor C 101 connected across the flashtube 102 is charged from a high voltage power supply 103 through a charging resistor R1 104. The capacitor 101 is often of large electrolytic type designed specifically for the rapid discharge needs of photoflash applications. The flashtube 102 remains non-conductive even when the capacitor 101 is fully charged.
In most cases a separate small capacitor Ct 105 can be charged from the trigger power supply 106 through a charging resistor R2 107. To generate a trigger pulse, the trigger source 109 is activated, the charge on the trigger capacitor 105 is dumped into the primary winding of a pulse trigger transformer 108 whose secondary is connected to a wire, strip, or a metal reflector in close proximity to the flashtube 102. The pulse generated by this trigger is enough to ionize the Xenon gas inside the flashtube 102 so that the Xenon gas suddenly becomes a low resistance and the energy storage capacitor 101 discharges through the flashtube 102, resulting in a short duration brilliant white light. Typical flash duration and intensity depends on the capacitance and the charge voltage of the storage capacitor 101. However, the cycle time is typically relatively much longer, of the order of a second, because of the time required to fully charge the energy storage capacitor.